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While traveling the lands in order to find a way to become a full-fledged human, Izumo learned of the Shikon no Tama, a jewel that purportedly had the ability to grant any wish made by its bearer. To his dismay, he discovered that the Jewel was shattered into many shards and that it would take a lifetime to reassemble. However, by forming an alliance with Orochidayū, a demon who also desired the Shikon's power and commanded hordes of lesser demons, Izumo devised a method to recreate the jewel again and again by harvesting the souls of numerous, random villagers. Though not nearly as powerful as the one made by Midoriko, the mass-produced Shikon Jewels could still empower demons considerably for a short period of time, but due to impurities, the imitations proved toxic to humans. Through more research, Izumo discovered that he could create a true Shikon Jewel by using the souls of those who possessed greater than normal amounts of courage, knowledge, love and friendship, which he found in Inuyasha, Miroku, Sango and Shippō respectively. He captured Kagome, planning to use her spiritual powers and three jewel shards to create the new Jewel.

In his final moments, Gyu-oh, desperate to create the new jewel before dawn (at which point he would become human again and therefore easily defeatable), swallowed a number of imitation Jewels to overcome Inuyasha. He was unable to defeat Inuyasha before daybreak, and when the sun rose, he reverted again to Izumo. With his human form unable to withstand the jaki of the fake jewels, he died, poisoned by his own creations. His lower half was unable to finish the transformation, leaving him a freakish satyr; his final words expressing regret that he was unable "to die as a human".

Gyu-oh was unique among half-demons. Instead of maintaining a half-human, half-demon appearance all the time as all other half-demons in the series are shown to do, he was fully human during the day and fully demonic at night. It was probably this trait that drove him into madness and fueled his desperation for the Shikon Jewel (and also explains his "unusual" obsession with the sun). While in human form, Izumo was pure--more so than normal humans to the extent that wild birds would land and rest on him. Yet in his demon form, Gyu-oh ironically seemed to have an affinity for the yokai crows much to his chargin.

High Intellect: Izumo is very intelligent and want to see the deeper meaning of some things. Kagome even said to Shippo that Izumo seems to be a scholar. His intellect allows great knowledge about the feudal era and the denizens in it that allows him to know how to make use of them.

Expert Manipulator: Izumo is very cunning and skilled at separating his human and demonic persona from each other during the day and night that allows him to deceive others to lure them into traps as he appears as two different people as shown when he lured Inuyasha and his group into an elaborate trap by taking advantage of human self and none of them realized it until he deliberately revealed himself.

Expert Jewel Craftsman: He is also a skilled craftsman of jewelry as he created multiple faux Shikon Jewels that while pales in comparison to the real one, has the ability to greatly enhance the strength of lesser demons though is highly toxic with miasma and the effects are only temporarily. He also knows exactly how to create another Shikon Jewel itself after studying extensively on Midoriko's legend of it's creation.

NocturnalTransformation: Izumo is rather unique for natural born half-demons as unlike most whom maintains a mixed appearance of human and demon features, he is fully human during day time and fully demon at night time. Izumo displays a 180 degree turn in persona whenever he transforms and considering he is completely human half of the day, it suggests unlike other half-demons he doesn't lose his demonic powers once a month though this has yet to be proven.

Enhanced Strength: In his demonic form as Gyu-oh, he displays impressive strength as he was capable of contending against the likes of Inuyasha in pure strength. He has also previously triggered a landslide with a single blow of his club.

Enhanced Durability: He has proved to be very durable as he was able to defend well against Inuyasha's attacks head on while the demons under his command were defeated fairly easily.

Enhanced Endurance: Gyu-oh has heightened stamina as he managed to fight against Inuyasha for a considerable amount of time while the lesser demons under his command were quickly dispatched even with the faux Shikon Jewels. The sun rose at the end which implies that their fight lasted several hours with neither tiring at all.

Miasma Generation and Manipulation: He has the ability to generate miasma from his very breath and manipulate it how he sees fit. Also he can resist the toxin of the faux Shikon Jewels after swallowing many of them to increase his power to overcome Inuyasha though only in his demon form as he died from the poisoning after becoming human at sunrise.

Great Demonic Power: Gyu-oh is rather powerful for a half-demon as during his full transformation he is capable of visibly intimidating lesser but full blooded demons into doing his bidding and through the enhancement of his faux Shikon Jewels proved to be considerable challenge for Inuyasha.

Barrier: When he was attacked by Inuyasha, Gyu-oh was protected by a barrier that was capable of warding off even the Tessaiga though it was unknown whether this was his own power or that of the faux jewels.

High Spiritual Power: As a priest and as Izumo, he possesses high spiritual energy and purity significantly more so than ordinary humans as even wild birds settle down an his hands and shoulders.

Izumo is the only half-demon born to a human father and a demon mother in the series.

He is also the only known half-demon who not only by his demonic parent has special powers, but also by his human parent (the intelligence and the spiritual purity).

Unlike Inuyasha, Jinenji and Shiori, one sees from his parents no flashback, so is unknown how they have looked.

Despite the common discrimination against half-demons, Orochidayū seemed to genuinely like Izumo/Gyu-oh and treated him with respect rather than grudging tolerance. Whether this was due to their mutually beneficial arrangement or because he was impressed by the other's cunning is unclear.